1. Field of the Invention
The present invention relates to the manufacture of fuel cell fluid flow plates with surface indentations, and more particularly, to the manufacture of such plates in a very efficient and cost effective manner.
2. Description of the Related Art
Fuel cells are electrochemical devices which directly combine hydrogen from a fuel and oxygen, usually from the air, to produce electricity and water. With prior processing, a wide range of fuels, including hydrogen, natural gas, methanol, gasoline and coal-derived synthetic fuels, can be converted to electric power. The basic process is highly efficient (80-90%), pollution-free, quiet, free from moving parts and may be constructed to leave only heat and water as by-products. Since single fuel cells can be assembled into stacks of varying sizes, systems can be designed to produce a wide range of energy output levels and thus satisfy numerous kinds of applications.
Fuel cell construction generally consists of a fuel electrode (anode) and an oxidant electrode (cathode) separated by an ion conducting layer. In operation, current is generated by a reaction on the electrode surfaces which are in contact with an electrolyte. Fuel and oxidant are supplied as required by the current load; and water is continuously removed. The electrode reactions are comprised of the oxidation of hydrogen on the anode to hydrated protons with the release of electrons. Stated in another way, the hydrogen gas molecules split into protons and electrons. On the cathode, the reaction is of oxygen with protons to form water vapor including a consumption of electrons. Electrons flow from the anode through the external load to the cathode and the circuit is closed by an ionic current transported through the electrolyte.
There are several different types of fuel cells under such labels as phosphoric acid, alkaline, molten carbonate, solid oxide and proton exchange membrane (PEM). The basic components of a PEM fuel cell are the two electrodes separated by a polymer membrane electrolyte. Each electrode is coated on one side with a thin platinum catalyst layer. The electrodes, catalyst and membrane together form a membrane electrode assembly. In a manner analogous to that described above, hydrogen fuel dissociates or splits into free electrons and protons in the presence of the platinum catalyst at the anode. The free electrons are conducted in the form of usable electric current through the external circuit. The protons migrate through the membrane electrolyte to the cathode. At the cathode, oxygen from air, electrons from the external circuit and protons combine to form pure water and heat. Individual fuel cells produce about 0.6 volts and are combined into a fuel cell stack to provide the amount of electrical power required.
Fuel cells may be used as stationary electric power plants in buildings and residences, as vehicle power sources in cars, buses and trucks and as portable power in video cameras, computers and the like.
A single fuel cell consists of a membrane electrode assembly and two fluid flow field plates. Hydrogen and air supplied to the electrodes on either side of the PEM through channels formed in the flow field plates. Hydrogen flows through the channels to the anode where the platinum catalyst promotes separation into protons and electrons. On the opposite side of the PEM, air flows trough the channels to the cathode where oxygen in the air attracts the hydrogen protons through the PEM. The electrons are captured as useful electricity through the external circuit and combine with the protons and oxygen to produce water vapor at the cathode side.
Reference is made to U.S. Pat. No. 5,300,370 (""370) issued in 1994 which describes a typical fuel cell fluid flow plate from 1984. The plate, in the form of a rigid electrically conductive panel, includes a plurality of parallel open-faced fluid flow channels formed in a major surface of the panel. The parallel channels extend between an inlet header and an outlet header formed in the panel. The parallel channels are typically rectangular in cross section and about 0.030 inches deep and about 0.030 inches wide. The inlet header is connected to an opening in the plate through which a pressurized reactant, either fuel or oxidant, is supplied. The outlet header is also connected to an opening in the plate through which the exhaust reactant and water are discharged from the cell. The reactant runs from the inlet to the inlet header and then to the parallel channels. The reactant then diffuses through a porous electrode material to the electro catalytically active region of the membrane electrode assembly. The reactant then flows to the outlet header and then to the outlet from which it is exhausted from the fuel cell. A plurality of continuous open-face fluid flow channels formed in the surface of the plate traverse the central area of the plate in a serpentine manner. This patent goes on to disclose that the fluid flow plates are made of graphite and the channels are milled, engraved or molded.
The ""370 patent discloses a new fluid flow field plate construction consisting of a stencil layer and a separator layer. The separator and stencil layers are formed of flexible graphite foil sheets having a thickness between about 0.003 inches and about 0.030 inches. Another patent, U.S. Pat. No. 5,521,018 (""018), discloses the concept of embossing a fluid flow field plate such as electrically conductive graphite foil sheet material. Other materials being sufficiently soft so as to permit embossing include porous electrically conductive sheet materials, such as carbon fibre paper, corrosive resistant metals, such as niobium; somewhat corrosive resistant material, such as magnesium or copper particularly when plated with noble metals such as gold or platinum to render them unreactive; and composite materials composed of corrosive metal powder, a base metal powder plated with corrosive resistant metal, and/or other chemically inert electrically conductive powders such as graphite and boron carbide bonded together with a suitable binder to produce a compressible electrically conductive sheet material. The embossing step is accomplished using a die where the channels are generally U-shaped or V-shaped in cross section. The ""018 patent discloses that xe2x80x9cthe graphite foil sheet is embossed at an embossing pressure sufficient to impart into the compressible sheet material, smooth-surface channels, of substantially uniform depth, and having a clean, reverse image of the embossing die. Different flow field patterns and plate sizes will require different embossing pressures. The bulk of the sheet material (that is, the portions of the sheet material located apart from the channels) can also be compressed during the embossing operation and the embossing pressure can be selected to provide the appropriate channel depth in cross sectional profile, and also to impart the appropriate electrical conductivity and porosity to the bulk material.xe2x80x9d
Still another U.S. Pat. No. 5,773,160 discloses the use of a coolant flow field plate in addition to a fuel flow field plate and an oxidant flow field plate. Yet another U.S. Pat. No. 5,981,098 (""098) issued in 1999 discusses fluid flow plates formed from a conductive material such as graphite where the flow channels are typically formed by machining. The patent also refers to an earlier fluid flow field plate comprising two outer layers of compressible electrically conductive material with an interposed center metal sheet. The outward faces of each of the two outer layers is embossed with flow field channels which are called xe2x80x9cindentationsxe2x80x9d. The ""098 patent goes on to describe fluid flow plates made by forming foil or sheet material into a design similar to a corrugation. Forming is accomplished by passing the plates between two rollers having patterns to make the channel grooves of preselected pitch and depth. One foil material is described as stainless steel. In this case, the height of the corrugated layer is 0.065 inches, with 32 channels per inch and a sheet thickness of 0.008 inches where the channels are 0.066 inches wide and 0.065 inches in depth. The plates may also be formed by stamping thin stainless steel sheet stock where the sheets have dimensions of 8.32 inches in length, 9.55 inches in width and 0.004 inches thick. The stamping is occasioned by a hydro-forming process in which each sheet is placed between an open dye and a piece of rubber that seals a high pressure oil chamber. Hydraulic pressure on the oil causes the rubber to impress or stretch the sheet as desired.
Still another U.S. Pat. No. 6,015,633 issued this year, discloses a fluid flow field plate having a thickness within the range of 0.020 to 0.300 inches with a preference for the range of 0.050 to 0.150 inches, where the channels have a width in the range 0.010 to 0.100 inches with a preference for the range 0.020 to 0.050 inches and a channel depth within the range 0.002 to 0.050 inches with a preference for the range of 0.010 to 0.040 inches. In addition, the cross sectional dimension of the width of the land separating adjacent channels is in the range of 0.010 to 0.100 inches and preferably within the range of 0.020 to 0.050 inches. The plate is described as a laminate with a generally non-porous planar base under a generally porous elongated strip. The non-porous portion may be comprised of a metallic material, such as stainless steel, or of a resin impregnated graphite material. The porous portion may be a wickey material, such as cotton cheese cloth. All of the above mentioned patents are incorporated herein by reference as are the references disclosed in each of them.
Methods and apparatus for embossing precision optical patterns in a resinous sheet or laminate is also well known, as referenced in such U.S. Pat. No. 4,486,363; 4,478,769; 4,601,861; 5,213,872; and 6,015,214 which patents are all incorporated herein by reference. By way of example, thin flexible thermoplastic material may be embossed with precision patterns where flatness and angular accuracy are very important. Products that require such accuracy include, for example, retroreflective materials for road reflectors or signage. As described in the above mentioned patents, the sheeting may be made on a machine that includes two supply reels, one containing an unprocessed web of thermoplastic material, such as acrylic or polycarbonate or even vinyl and the other containing a transparent plastic carrier film such as Mylar. These are fed to an embossing tool which may take the form of a thin endless metal belt.
The belt moves around two rollers which advance the belt at a predetermined linear speed or rate. One of the rollers is heated and the other roller is cooled. An additional cooling station may be provided between the two rollers. Pressure rollers are arranged about a portion of the circumference of the heated roller. Embossing occurs on the web as it passes around the heated roller and while pressure is applied. The embossed, now laminated sheeting, is monitored for quality and then moved to a storage winder. Before shipping the Mylar film may be stripped away from the embossed film.
The embossing tool may be made by electroforming as described in U.S. Pat. Nos. 4,478,769 and 6,015,214. The design to be embossed on the sheeting begins by forming the design on specific plates made of one of a number of specified materials including electroless nickel. These plates are replicated to produce a flexible strip having an uninterrupted pattern. The strips are assembled on a cylindrical mandrel to provide cylindrical segments. The cylindrical segments are assembled to provide a cylinder of the desired dimensions corresponding to the width of the web intended to be provided with rectroreflective elements. The assembled cylinder is used to form a flexible endless master cylinder having the pattern of microcubes. The master cylinder is then used to form a relatively thick mother cylinder which in turn is used to form a generally cylindrical metal embossing tool.
The embossing tool may then be used to emboss the microcubes on a surface of a continuous resinous sheeting material to manufacture a rectroreflective sheeting article as described in U.S. Pat. No. 4,486,363 which has been briefly described hereinabove.
Continuous press machines are also well known. These include double band presses which have continuous flat beds with two endless bands or belts, usually steel, running above and below the product and around pairs of upper and lower drums or rollers. These form a pressure or reaction zone between the two belts and have the advantage that pressure is applied to a product when it is flat rather than when it is in a curved form. The double band press also allows pressure to vary over a wide range and the same is true about temperature variability. Dwell time or time under pressure is also easily controllable by varying the production speed or rate, as is capacity which may be changed by varying speed and/or length of the press.
In use, the product is xe2x80x9cgrabbedxe2x80x9d by the two belts and drawn into the press at a constant speed. At the same time, the product, when in a relatively long flat plane, is exposed to pressure in a direction normal to the product. Of course, friction is substantial on the product but this may be overcome by one of three systems. One system is the gliding press, where pressure-heating plates are covered with low-friction material such as polytetrafluorethylene and lubricating oil. Another is the roller bed press, where rollers are placed between the stationary and moving parts of the press. The rollers are either mounted in a fixed position on the pressure plates or incorporated in chains or roller xe2x80x9ccarpetsxe2x80x9d moving inside the belts in the same direction but at half speed. The roller press is sometimes associated with the term xe2x80x9cisochoricxe2x80x9d. This is due to the press providing pressure by maintaining a constant distance between the two belts where the product is located. Typical isochoric presses operate to more than 700 psi.
The third press type is the fluid or air cushion press which uses a fluid cushion of oil or air to reduce friction. The fluid cushion press is sometimes associated with the term xe2x80x9cisobaricxe2x80x9d and these presses operate to about 1000 psi. Pressure on the product is maintained directly by the oil or the air. Air has the advantage of providing a uniform pressure distribution over the entire width and length of the press.
Heat is transferred to thin products from the heated rollers or drums via the steel belts. With thicker products heat is transferred from heated pressure plates to the belts and then to the product. In gliding presses, heat is also transferred by heating the gliding oil itself. In roller bed presses, the rollers come into direct contact with the pressure-heating plates and the steel belts. With air cushion presses, heat flows from the drums to the belts to the product, and, by creating a turbulence in the air cushion itself, heat transfer is accomplished relatively efficiently. Also, heat transfer increases with rising pressure.
Another advantage of the double band press is that the product may be heated first and then cooled with both events occurring while the product is maintained under pressure. Heating and cooling plates may be separately located one after the other in line. The steel belts are cooled in the second part of the press and these cooled belts transfer heat energy from the product to the cooling system fairly efficiently.
Continuous press machines fitting the description provided hereinabove are sold by Hymmen GmbH of Bielefeld, Germany (U.S. office: Hymmen International, Inc. of Duluth, Ga.) as models ISR and HPL. These are double belt presses and also appear under such trademarks as ISOPRESS and ISOROLL. Typically they have been used to produce relatively thick laminates, primarily for the furniture industry.
Even though fuel cell fluid flow field plates are known, as are their present manufacturing techniques, improvements are still needed to increase manufacturing efficiency, improve quality and lower cost.
The present invention relates to a process for manufacturing fuel cell plates with increased efficiency, improved quality and lower cost. What is described here is a process for making fuel cell plates comprising in combination the steps of providing a continuous press with a movable belt, dispensing a sifted material onto the belt, leveling the material, removing air from the material, subjecting the material to pressure and heat, and indenting the material with a predetermined pattern.
An object of the present invention is to provide an efficient manufacturing apparatus and process for making fuel cell plates. A further aim of the present invention is to provide an efficient and cost effective method and apparatus for making indentations in resin impregnated graphite material.
A more complete understanding of the present invention and other objects, aspects, aims and advantages thereof will be gained from a consideration of the following description of the preferred embodiments read in conjunction with the accompanying drawings provided herein.